In some conventional image-forming devices, an electrostatic latent image formed on a photoreceptive drum is developed and made visible by affixing toner thereto, and the toner image thus formed is transferred to a transfer material wrapped around a transfer drum.
In this type of image-forming device, as shown, for example, in FIG. 10, inside a drum 101 having a dielectric layer 101a are separately provided a corona electrical charger 102, for affixing a transfer sheet P to the drum 101, and a corona electrical charger 104, for transferring to the transfer sheet P a toner image formed on a photoreceptor drum 103. Thus affixing of the transfer sheet P and transfer of the toner image to the transfer sheet P are performed separately, by the corona electrical chargers 102 and 104, respectively.
Again, some image-forming devices, as shown in FIG. 11, are provided with a drum 201 with a two-layer structure of an outer semiconducting layer 201a and an inner base material 201b, and with a gripping structure 202, for maintaining a transfer sheet P in contact with the drum 201. In this image-forming device, the gripping structure 202 grasps one end of the transfer sheet P and brings it into contact with the surface of the drum 201. Then the surface of the drum 201 is given a charge by application of a voltage to the outer semiconducting layer 201a or by discharge of an electrical charger provided inside the drum 201. In this way, the toner image formed on the photoreceptor drum 103 is transferred to the transfer sheet P.
However, in the image-forming device shown in FIG. 10, the drum 101, which is a transfer roller, has a single-layer structure of the dielectric layer 101a only. Therefore, the corona electrical chargers 102 and 104 must be provided inside the drum 101. This places restrictions on the size of the drum 101, creating the problem that the size of the device as a whole cannot be reduced.
In the image-forming device shown in FIG. 11, the two-layer structure of the drum 201 is used to give the drum 201 the charge necessary to transfer the toner image to the transfer sheet P. Therefore, in this image-forming device, the number of chargers can be reduced. However, provision of the gripping structure 202 makes the structure of the image-forming device as a whole more complex. This leads to problems such as increase of the number of parts in the device as a whole and of the cost of manufacture.
In order to solve the foregoing problems, Unexamined Japanese Patent Publication No. 74975/1990 (Tokukaihei 2-74975), for example, discloses an image-forming device in which a corona electrical charger driven by a unipolar power source is provided near the point where a transfer material separates from a transfer drum made up of conductive rubber and a dielectric film layered on a grounded metal roll. In this image-forming device, a charge is induced in the conductive film by the corona electrical charger, thus affixing the transfer material to the transfer drum. After the transfer material is affixed to the transfer drum, a further charge is induced, causing transfer to occur.
Accordingly, in the above image-forming device, the affixing of the transfer material and the transfer of the toner image carried out by charging the surface of the transfer drum can both be carried out by a single charger. As a result, the transfer drum can be reduced in size. Further, there is no need for a structure like the gripping structure 202 to hold the transfer material, and the transfer material can be affixed by means of a simple structure.
Further, U.S. Pat. No. 5,390,012 discloses a transfer device provided with a transfer drum having at least an elastic layer made of a foam material and a dielectric layer covering the elastic layer, in which single-color toner images successively formed on a photoreceptor drum are successively transferred to a transfer material affixed to the transfer drum, thus forming a full-color image on the transfer material.
In this transfer device, the transfer material is electrostatically affixed to the transfer drum using an affixing roller as charge applying means. Further, by providing a gap of 10 .mu.m or more between the elastic layer and the dielectric layer, a charge is allowed to build up on the reverse side of the dielectric layer (the side away from the transfer material). As a result, the potential of the dielectric layer can be maintained without being influenced by the environment, thus improving the affixing of the transfer material to the transfer drum. Also disclosed is a method of creating an electric field necessary to affix the transfer material to the surface of the transfer drum by scattering insulator particles in the gap between the elastic layer and the dielectric layer.
Although it is not disclosed in the foregoing, a method providing an intermediate resistor between the dielectric layer and the elastic layer is also possible. In this case, the change of the electric field due to the gap between the elastic layer and the dielectric layer will be as small as possible.
Further, Japanese Examined Patent Publication No. 84902/1993 (Tokukohei 5-84902) discloses a multi-layered transfer device having a transfer drum for transferring a toner image formed on a photoreceptor drum to a transfer material at a transfer point. On the transfer drum is layered a dielectric layer with a dielectric constant of 3.0 to 13.0, a thickness of 70 .mu.m to 200 .mu.m, and a critical surface tension of no more than 40 dyne/cm. In this multi-layered transfer device, transfer performance in an environment or ambient atmosphere is maintained by the electrical characteristics of the dielectric layer described above. Further, cleaning of the transfer drum after separation of the transfer material is ensured by the critical surface tension mentioned above.
Further, a transfer drum with the structure shown in FIG. 12, in which a semiconducting layer 302 and a dielectric layer 303 are layered, in that order, on the surface of a conductive layer 301 made of aluminum, etc., has also been proposed. In a transfer drum of this type, the semiconducting layer 302 is made of a foam material which is a mixture of, for example, EPDM (ethylene-propylene-diene co-polymer) and conductive particles, a foaming agent, etc. As a result, a plurality of tiny bubbles are formed within the semiconducting layer 302, and these bubbles give the surface of the transfer drum a cushion. Further, when a voltage is applied to the conductive layer 301, giving it a potential difference from a ground roller (not shown), a discharge effect arises in these bubbles. This discharge causes a charge to arise on the reverse side of the dielectric layer 303 (the side toward the semiconducting layer 302), which gives rise to a strong affixing force with respect to the transfer material.
With the structure according to Unexamined Japanese Patent Publication No. 74975/1990 (Tokukaihei 2-74975), charging of the surface of the transfer drum is performed by atmospheric discharge from the corona electrical charger. As a result, when transfer is to be carried out a number of times, as for instance in color copying, the charge must be replenished by the corona electrical charger after each transfer. Accordingly, a charging unit composed of a unipolar power source, etc. becomes necessary to control driving of the corona electrical charger. This gives rise to problems such as increase of the number of parts in the device and of the cost of manufacture.
Further, since the surface of the transfer drum is charged by atmospheric discharge, any scratch or nick in the surface of the transfer drum will reduce the electric field area. As a result, the electric field balance will be disturbed at the scratch or nick, giving rise to transfer failure such as a white spot at that point, and to diminished image quality. Further, with atmospheric discharge, the voltage required to charge the surface of the transfer drum is large, and the energy necessary to drive the image-forming device is increased. Atmospheric discharge is also easily influenced by environmental factors such as air temperature and humidity, and changes in the environment can give rise to uneven potential in the surface of the transfer drum. This can result in problems such as insufficient affixing of the transfer material, distortion of printed letters, etc.
Again, in the structure according to U.S. Pat. No. 5,390,012, a gap is provided between the elastic and dielectric layers making up the transfer drum. As transfer is performed repeatedly, the form of the dielectric layer is repeatedly changed each time a nip is formed between the dielectric layer and the photoreceptor, and the gap becomes larger over time. In other words, uniformity cannot be maintained in the size of the gap (which is distinct from the dielectric layer) formed between the foam elastic layer and the dielectric layer. Nor does the resistance of the elastic layer remain constant over time. As a result, image quality deteriorates as transfer is performed repeatedly. In order to maintain uniformity of the size of the gap and the resistance of the dielectric layer, the structure of the transfer device becomes complicated, giving rise to the problem of increase of the manufacturing cost of the device as a whole.
Further, the disclosure cited above does not stipulate the hardness of the elastic layer or the contact pressure between the charge-applying means (affixing roller) and the transfer drum. Nor does it discuss the width of the nip between the charge-applying means (affixing roller and bias voltage applying method) and the transfer drum, or the nip time. In other words, the nip time is apparently fixed, regardless of the type of transfer material.
It is well known that the amount of charge injected into a transfer material during a constant nip time generally varies according to the transfer material used. A transfer drum's ability to electrostatically affix a transfer material to the dielectric layer is also dependent on the transfer drum's hardness, i.e., the amount of elastic change in its form. Accordingly, with the structure according to the disclosure cited above, the ability of the transfer drum to perform transfer by electrostatic charge may be impaired, depending on the type of transfer material used. This results in the problem of poor transfer of the toner image from the photoreceptor drum to the transfer material. Further, with this method, at least two power sources are required: an affixing roller power source for affixing the transfer material to the transfer drum, and a power source for applying to the transfer material at the time of toner transfer a voltage of reverse polarity with respect to the toner. This results in the problem of increase of the number of parts and the size of the device as a whole.
Further, since a foam material is used to provide the gap, there are cases, depending on the quantity of toner at the time of transfer, when the pattern of the foam shows in the printed letters. As a method of resolving this problem caused by the gap, the LBP2030 image-forming device manufactured by Canon Co., Ltd., for example, provides an intermediate resistance coating on the reverse side of the dielectric sheet used as the surface layer of the transfer drum. By this means, the local differences in electric field which arise due to the gap of the elastic layer are brought into uniformity.
However, with this type of full-color printer, which is already on the market, it is difficult to stably hold the transfer material by electrical attraction alone, and a transfer material gripper, etc. becomes necessary to hold the transfer material. This results in the problem of increase of the number of parts and of the size of the device as a whole.
Again, in the transfer drum structure shown in FIG. 12, the air bubbles within the semiconducting layer 302 are provided with a substantially uniform size. As a result, image quality deteriorates in both high-temperature, high-humidity and low-temperature, low-humidity operating environments.
In order to satisfy both solid/halftone transfer and letter transfer, it is necessary to increase the hardness of the transfer drum by uniformly reducing the diameter of the foam particles. However, if the diameter of the foam particles is uniformly reduced, a phenomenon occurs under high-temperature, high-humidity conditions in which some of the lines making up printed letters are not printed, thus impairing image quality, and affixing of the transfer material using electric lines of force is also diminished.
If, on the other hand, the hardness of the transfer drum is reduced by uniformly increasing the size of the foam particles, white spots, scattering, etc. occur in the printed image under low-temperature, low-humidity conditions due to the bubbles within the foam area, which markedly diminish image quality.
It has been experimentally found that with foam particles approximately 1 mm in diameter, white spots are clearly visible even in solid transfer, and that with foam particles 500 .mu.m or more in diameter, white spots occur in halftone transfer.
Accordingly, with regard to image-forming devices in which a toner image is transferred from a photoreceptor to a transfer material while the transfer material is electrostatically affixed and held to the surface of a transfer drum, various operating conditions such as high-temperature, high-humidity and low-temperature, low-humidity conditions need to be taken into consideration. However, in the transfer drum structure discussed above, since the foam particles in the semiconducting layer 302 are provided with a substantially uniform size, image quality is diminished in both high-temperature, high-humidity and low-temperature, low-humidity conditions. As a result, this image-forming device has the shortcoming that insufficient affixing of the transfer material, distortion of printed letters, deterioration of image quality, etc. are likely to occur.
In order to avoid white spots, etc., a conductive film (approx. 8 .OMEGA./cm to 9 .OMEGA./cm) could be provided between the semiconducting layer 302 and the dielectric layer 303 of the transfer drum. However, in this case the affixing of the transfer material is markedly impaired, making a transfer material gripper necessary to hold the transfer material, and thus increasing the size of the device as a whole.